1. Field of the Invention
The present invention relates to a method of and an apparatus for manufacturing a thin solar battery, and in particular to a method of accurately and easily forming an electrode on one side of a power generation layer, that is, a semiconductor film, a method of treating the semiconductor film for this purpose and a thin solar battery manufacturing apparatus which treats a semiconductor film.
2. Description of the Background Art
Solar power generation systems for ordinary houses are marketed increasingly faster these days. Under the circumstances, it is a challenge for manufacturers to manufacture a solar battery cell and a solar battery module for use in solar power generation at a low cost without deteriorating photoelectric conversion efficiency of the battery. Considering the contents of a manufacturing cost of a solar battery module, it is understood that a silicon material from which the battery is basically manufactured and a cost for assembling the module are expensive. Hence, a cost reduction in these two items is generally regarded as the fastest way to realize a reduction in the total manufacturing cost.
Meanwhile, in a step for assembling a conventional solar battery cell as a module which is disclosed for example in "Solar Battery Handbook," Institute of Electrical Engineers, Solar Battery Committee, Chap. 6 Modularizing Technique, pgs. 163-167 (1985, First edition, First print), connection of cells using element link lines called "tab" is complex. This will be described in the following with reference to FIG. 50A which shows a step of modularizing a conventional solar battery (cell). In the step shown in FIG. 50A, tab electrodes 73 are formed on the both sides of each silicon substrate 74 (typically of a thickness equal to or more than 350 .mu.m) which will serve as a power generation layer of the solar battery. Next, at the step shown in FIG. 50B, solar battery cells are arranged for serial connection to each other and the tab electrode 73 of each solar battery cell is soldered to the tab electrode 73 of an adjacent solar battery cell. Following this, at the step shown in FIG. 50C, a transparent sheet 8 such as EVA (ethylene vinyl acetate) is placed on a tetrafilm 75, i.e., a fluoride vinyl film in which an aluminum foil is sandwiched, and the silicon substrates 74 arranged and interconnected at the step shown in FIG. 50B are placed on the transparent sheet 8, and further another transparent sheet 8 is placed on the tetrafilm 75, and finally a glass 7, i.e., a modularizing structure member which serves also as a window member, is stacked on the upper transparent sheet 8. At the step shown in FIG. 50D, the stacked structure is heated to about 150.degree. C. in a bonding apparatus for deaeration between the modularizing glass 7 and the tetrafilm 75. Further, at the step shown in FIG. 50E, the stacked structure is framed into a frame 76 made of aluminum, an output terminal 77 is attached to the stacked structure using a silicon resin and a back plate 78 is fit in the stacked structure, thereby completing modularizing.
Electrodes are formed on the both sides of the silicon substrate in the conventional solar battery as described above. Hence, connection between the cells requires connecting a tab which is formed on a back side of each cell with the cell surface of the adjacent cell which is arranged at a desired position. This step is very complicated. In addition, since the modularizing steps are as described above, the step of stacking the elements ranging from the tetrafilm to the modularizing glass and the subsequent deaeration step are complex, and therefore, a reduction in the assembling cost necessary for these steps is difficult. Further, since a silicon substrate is used as a basic body of a solar battery as described earlier, it is difficult to reduce a cost for a silicon material. Although it is said that a solar battery using a crystal silicon wafer needs be as thick as 500 .mu.m in general, in reality, the thickness does not need to be 100 .mu.m or larger for absorption of solar light. Rather, if incident light is to be contained within a power generation member, that is, light containment is to be efficient so that an optical length of light having a long wave length and a small absorption coefficient becomes sufficiently long, since carriers created by the incident light within the power generation member are efficiently collected when the thickness of the solar battery is thin, the solar battery is preferably thin so as to achieve excellent performance. In other words, a reduction in the thickness of a semiconductor portion which serves as the power generation layer is advantageous from two points of view, one being a reduction in the manufacturing cost (material cost) and the other being the photo-conversion efficiency.
To this end, a thin solar battery has been proposed which has a cell structure in which a thin semiconductor film thinner than at least a silicon substrate is formed on the silicon substrate and separated from the silicon substrate and a desired electrode is formed on the semiconductor film. FIGS. 51A to 51E are views describing a conventional thin solar battery as that disclosed by Japanese Patent Laid-Open Gazette No. 4-333288 along with method of manufacturing the same. In FIGS. 51A to 51E, denoted at 101 is a heat-resistant substrate formed by a silicon wafer, denoted at 111 is an insulation layer which is formed by a silicon oxide film, denoted at 112 is a first silicon layer having a small resistance, denoted at 113 is a second silicon layer having a large resistance, denoted at 114 is a gap which is locally created in the insulating layer 111, denoted at 106 is a grid electrode, and denoted at 107 is a back surface electrode.
First, the insulation layer 111 is formed selectively on the heat-resistant substrate 101 (FIG. 51A). On an exposed area of the heat-resistant substrate 101 formed by a silicon wafer where the insulation layer 111 is not formed, the first silicon layer 112 and the second silicon layer 113 are selectively and epitaxitially grown in this order (FIG. 51B). At this stage, a silicon film is not formed on the insulation layer 111. Instead, the gap 114 is formed locally on the insulation layer 111. Through this gap 114, the insulation layer 111 is etched using hydracid fluoride (FIG. 51C). Following this, a mixture of hydracid fluoride, nitric acid and acetic acid is injected through a space where the gap 114 and the insulation layer 111 were removed so that the first silicon layer 112 is selectively etched and the second silicon layer 113 is separated from the heat-resistant substrate 101 due to a difference in etching speeds which is created by a difference in specific resistances (FIG. 51D). A bonding layer 105 is then formed, and the grid electrode 106 and the back surface electrode 107 are formed on the bonding layer 105 by sputtering of metal or other technique (FIG. 51E).
In the manufacturing method of the thin solar battery as above, processes such as selective etching of the first silicon layer 112 and selective epitaxial growth of the semiconductor film are not sufficiently reliable for mass production, which makes it difficult to reduce an overall cost. Further, even though the thin semiconductor film is used to improve the photo-conversion efficiency, since the electrodes are formed on the both sides of the semiconductor layer, the utilization efficiency of manufacturing light deteriorates because of the surface area which is used aiming to seat the electrodes on the light incident side.
A specific solar battery structure as that shown in FIG. 52 has already been proposed in Patent Laid-Open Gazette No. 6-053782 to deal with such problems. FIG. 52 is a cross sectional view partially showing a structure of this solar battery. In FIG. 52, a semiconductor film 2 is formed by a p-type polycrystal silicon film having a thickness of 60 .mu.m and a specific resistance of about 1 .OMEGA.cm, for instance. A bonding layer 122 is formed by diffusing n-type impurities such as phosphorus into the semiconductor film 2. The bonding layer 121 is also formed inside a through hole 114 which is formed in the semiconductor film 2. Electrodes for the bonding layer 122 may be a first electrode (n-type electrode) 127 made of silver or the like and a second electrode (p-type electrode) 128 made of aluminum or the like which is formed in a p-type area where the bonding layer 122 is not formed. Generated electricity is available outside through the electrodes 127 and 128. The semiconductor film 2 is adhered to a glass substrate 129 through a transparent adhesive 121. Light impinges upon from the glass substrate side. In a cell, i.e., a unit of this solar battery, the bonding layer is formed at least on a major surface of the semiconductor film 2 of the light incident side and on an inner wall of a through hole formed in the semiconductor film (for separation). Further, the first electrode 127 connected to the bonding layer is formed on the opposite side of the major surface of the semiconductor film. Hence, loss of the light receiving area due to the existence of the first electrode is less, whereby light energy is utilized efficiently. In addition, since the cell is not more than 100 .mu.m in thickness, a material cost is largely reduced than in manufacturing of the conventional cell. The n-type and p-type electrodes 127 and 128 are formed on the opposite side of the light incident side, and therefore, in the case of connecting by forming tab electrodes, it is not necessary to connect the tabs from the front surface side to the back surface side of adjacent cells unlike in the conventional techniques. As a result, the connection step is largely simplified. Still further, Japanese Patent Laid-Open Gazette No. 6-053782 has already disclosed a method of separating a semiconductor film from a specific substrate, a method of forming a through hole in a semiconductor film, a method of forming p-type and n-type bonding layers and other methods.
The modularizing as above using a thin solar battery cell realizes both a higher efficiency owing to an increased light receiving area and a suppressed cost owing to a reduced material cost of a silicon material and simplified connection performed at the tab electrode forming step. While formation of electrodes on each cell is achieved by vacuum deposition or sputtering evaporation for each cell or by global printing with the cells arranged on a plane for modularizing, in global printing which efficiently forms electrodes, as the light receiving area increases, it becomes more difficult to ensure that the accuracy of printed masks, the location of an electrode to be printed and the configuration of each electrode does not exceed 100 .mu.m.
Further, even though connection step is simplified at the tab electrode forming step, it is still necessary to repeat the step of forming electrodes for each cell, the step of forming tab electrodes for each cell and the step of connecting the cells to each other. For this reason, simplification of these steps has been desired.
Still further, as a solar battery cell becomes thinner, it became more difficult to handle a solar battery through the process of manufacturing a cell and the modularizing processes. As a result, due to damage by dropping and inadvertent handling, a deteriorated yield has became a problem. Handling requires an extremely long time to prevent such damage, which serves as an obstacle to improve the productivity.